The intense interest in sports, exercise and fitness training has produced a vast literature that describes all the events that occur during an individual's period of exercise. Not all the studies are in agreement, but basic changes that occur in all individuals have been established by a variety of techniques including the direct biopsy of muscle tissue and blood sample analysis. Oxygen consumption and CO2 production are fundamental quantities both at rest and during exercise. However, these parameters are difficult and inconvenient to ascertain during exercise. Therefore, reliable, easy to measure surrogate parameters are needed.
If energy production by muscles during exercise kept pace with energy demand, then the complete oxidation of glucose and fatty acids as the major sources for energy would continue. However, it was found in early studies that during intense exercise there is a progressive increase in lactic acid in muscles, some of which is transported into plasma. This build-up in lactic acid, often referred to as anaerobic metabolism or glycolysis, represents an imbalance between energy demand and energy supply that may be restored by completing its oxidation at a later time.
A comprehensive review discusses whether lactic acid production during exercise should be viewed as a continuous function or a threshold, based on the interpretation of mathematical models (Myers and Ashley, Chest 111:787-95 (1997)). Nevertheless, it is clear from plasma lactate analysis that sharp inflection points do occur in the later phases of maximum exercise.
Radioactive and/or stable isotopes of sodium bicarbonate have been used previously for evaluating exercise performance (Speakman and Thomson, Zeitschrift fur Ernahrungswissenschaft 36:273-7 (1997)); however, the use of the isotopes of sodium bicarbonate did not permit a clear distinction between CO2 production attributable to buffering and CO2 produced that related only to energy production. Therefore, it was not possible to distinguish between aerobic and anaerobic metabolism during exercise and during the post-exercise period. As such, methods are needed to distinguish between the CO2 produced because of lactate production from that generated by complete oxidation, which, in turn, would allow monitoring of the proportion of aerobic and anaerobic metabolism during exercise.